The present invention is related to electrically driven motor vehicles, and in particular motor torque control during shifting of a multi-speed transmission coupled between the motor output and the drive wheels.
Given that the speed of the output member of a multi-speed transmission remains substantially constant during a gear change, it necessarily follows that the input member of the transmission must undergo a substantial speed change in order to complete the gear change. In an upshift, the input member of the transmission experiences a deceleration to a lower speed. Therefore, the prime mover coupled to the input member of the transmission also undergoes a substantial decelerating speed change in effectuating such a gear change. In a downshift, the input member of the transmission experiences a acceleration to a higher speed. Therefore, the prime mover coupled to the input member of the transmission also undergoes a substantial acceleration or deceleration in effectuating a gear change.
Generally, the torque produced by an internal combustion engine varies significantly with engine speed. The torque coupled to the input member of the transmission immediately following an upshift gear change in a vehicle powered by an internal combustion engine therefore is typically lower than that coupled to the input member immediately prior to the upshift. The output torque is also characteristically lower after the upshift is complete than it is prior thereto, due in part to the lower input torque and to the lower ratio associated with the new gear. These are well known mechanics of a power-on upshift gear changes. Vehicle operators have become accustomed during power-on upshifts to increasing the accelerator pedal position in an attempt to maintain the level of vehicle performance (i.e. output torque) in the new gear as was present in the old gear. Correspondingly similar occurrences are found with power-on downshifts, however with respect to higher output torques and operator responses thereto.
On the other hand, electric traction motors are known to be able to produce torque output substantially independent of operating speed with appropriate motor current at least within a range of operating speeds below a base speed. Electric vehicles are known having a traditional accelerator pedal for controlling the motor output and vehicle performance in a manner analogous to an internal combustion powered vehicle. However, accelerator pedal position as associated with an electric vehicle application conventionally is used to control motor current. Upshifting in an electrically-driven vehicle having an automatic multi-speed transmission may result in an undesirable driveline sag once the speed change has occurred. After the speed change, the motor will be operating at a lower speed and, if the accelerator pedal position remains substantially unchanged from its pre-shift position, motor current too remains unchanged. The torque output of the motor will be substantially the same as before the speed change and the output torque or wheel torque will decrease in correspondence with the difference between the old and new speed ratios. Correspondingly similar occurrences are found with power-on downshifts, however with respect to higher output torques.